The recent growth of power circuit capacities has caused fault currents to increase. Since the protection of power systems from the fault currents is very important, it is needed to develop a fault current limiter. A fault current limiter is required to assure (1) rapid reaction to fault currents, (2) how impedance in normal operation and (3) large impedance during fault conditions. A super conducting fault current limiter (SCFCL) can meet these requirements superconductors, because of their sharp transition from zero resistance at normal current to finite resistance at higher current densities, are tailor-made for use in FCLs.
Super conductors are of two types-high temperature superconductors (HITS) and low temperature superconductor (LTS). The HTS are substances that lose all resistance below temperature main tamable by liquid nitrogen. LTS are substances that lose all receptivity close to 4k, a temperature attainable only using by using liquid helium. Cost of cooling LTS (which are mostly metals, alloys and intermettalics) makes their use in many applications commercially impractical. HTS material available are all made of bismuth (BSCCO) or yttrium-cup rate (YBCO). So far, various types of SCFLS have been developed (resistance, shield core type, hybrid etc.). The SCFCL offers efficient advantages to power system and opens up a major application for superconducting materials.
Damage from a short circuit is a constant threat to any electric power system. Insulation damaged by aging an accident or lightning strike can unloose immense fault currents practically the only limit on their size being the impedance of the system between their location and power sources. At their worst, faults can exceed the largest current expected under normal load – the nominal current by a factor of 100 producing mechanical and thermal stresses in proportion to the square of the current’s value.
All power system components must be designed to withstand short circuit stresses for certain period determined by time needed for circuit breakers to activate (20-300 ms). The higher the fault currents anticipated the higher will be the equipment and also the maintenance cost. So there obviously is a big demand for devices that under normal operating conditions have negligible influence on power system but in case of fault will limit the prospective fault current. A device of this kind is called fault current limiter.